1. Field of the Invention
The present invention generally relates to an orthogonal frequency division multiplexing (OFDM) receiving apparatus for use in a base station or a terminal station of a radio communication system that performs OFDM radio transmission, or to an OFDM receiving apparatus for use in a receiving station of an OFDM broadcasting system. More particularly, the invention relates to an OFDM diversity receiving apparatus that implements high-quality information transmission even under a poor radio propagation environment.
2. Description of the Background
Terrestrial television broadcasting currently use analog signals. There are plans, however, in about the year 2000, for analog signals to be replaced by digital signals in terrestrial television broadcasting. In terrestrial television broadcasting, the area covered by a single transmission station is very large. A serious problem thus encountered is that multiple reflection wave transmission (multipath) occurs in propagating radio waves, thus degrading image quality. This is referred to as xe2x80x9cghost interferencexe2x80x9d. The scale of the multipath in terrestrial television broadcasting is much greater than that occurring in radio communication system, such as mobile cellular telephones. Even using an adaptive equalizer, which should be effective as a countermeasure against multipath, the multipath in terrestrial television broadcasting can no longer be handled.
In view of the above background, the OFDM transmission method, which is, in principle, resistant even under a poor multipath transmission environment and achieves high-quality information transmission, is to be employed as a digital transmission method for terrestrial television broadcasting. An OFDM transmission signal is provided with a guard interval in which part of a transmission waveform (symbol) is copied. The guard interval accommodates multipath propagation which is shorter than the length of the guard interval, thereby preventing fatal degradation of the image quality.
There are high expectations that the above-described multipath-resistant transmission characteristics of the OFDM transmission method will be used not only in terrestrial television broadcasting, but also in wide-area radio communication systems, such as multimedia communications, which are to be widely performed in public networks. Accordingly, a technical study of the practical application of the above OFDM transmission method is actively in progress.
Additionally, according to the OFDM transmission method, by virtue of the multipath-resistant transmission characteristics, a single frequency network (SFN) in which the same information is simultaneously transmitted with the same frequency can be constructed. This makes it possible to prevent wasteful use of a frequency, and more specifically, to prevent the transmitting frequency from being changed according to the area, which is conventionally required. In terms of compressing a frequency band for use, the OFDM transmission method is very effective.
However, even though the OFDM transmission method exhibits excellent multipath-resistant transmission characteristics, the receiving characteristics cannot be perfectly maintained because of severe frequency selective fading caused by multipath propagation, or Doppler shift and time fading occurring while a receiving end is moving.
High-definition image transmission is strongly expected to be achieved in digital terrestrial television broadcasting and the next-generation multimedia communications. Particularly in this type of transmission, there is a demand for a radio transmission technique which is much more stable and which exhibits higher quality transmission than that being used in current mobile cellular telephones, in which sound communications rather than image communications are the mainstream. Accordingly, the advent of a receiving method and a receiving apparatus that exhibit higher-quality receiving characteristics is greatly needed.
High-definition images having a large amount of information are required to be transmitted in a narrow radio band in order to effectively and efficiently use the frequency band. It is thus necessary to consider the use of a high-efficiency modulation method, such as a multilevel quadrature amplitude modulation (QAM), under a mobile radio propagation environment. However, a high-efficiency modulation method represented by QAM has poor noise-resistant characteristics and interference-resistant characteristics, and is also vulnerable to distortions.
In a receiving apparatus positioned far from a transmission station or a base station, or in a receiving apparatus receiving signals while it is moving, the received signals have a low signal-to-noise (S/N) ratio or have a propagation distortion. This easily causes degradation of receiving characteristics and consequent, failure to transmit information with a satisfactory quality. This problem becomes serious particularly for terrestrial television broadcasting that covers a very wide area. Accordingly, an effective improvement in the transmission method is required.
A typical measure being taken for improving the receiving characteristics under a severe multiple radio propagation environment or under a poor mobile receiving environment is diversity reception. As a diversity reception method, antenna selection diversity reception is primarily used in which digital signal sequences are demodulated by selecting from a plurality of receiving antennas the receiving antenna having the maximum radio frequency (RF) signal power.
In the OFDM transmission method, however, a received signal is frequency-converted into a baseband signal, which is further converted into frequency spectra in units of symbols. A digital signal sequence is then demodulated from each line spectrum which forms a frequency spectrum. Thus, the antenna selection diversity reception that changes antennas by RF signals is not very effective on the OFDM transmission method. Particularly upon generation of severe frequency selective fading, the receiving characteristics are completely different according to the line spectrum which forms a frequency spectrum. Thus, there is an increasing demand for selection diversity that obtains optimal receiving characteristics for each line spectrum.
As discussed above, in a radio communication/broadcasting system for transmitting digital signal sequences by using the OFDM method, it is necessary to transmit high-quality and high-definition information in all the places within a wide area covered by this system while effectively utilizing the multipath-resistant transmission characteristics of the OFDM transmission method. To satisfy the above requirements, it is further necessary to inhibit the degradation of the receiving characteristics under a severe multiple reflection wave propagation (multipath) environment and to suppress the degradation of the receiving characteristics while a receiving apparatus is moving.
In particular, in future multimedia communications and digital terrestrial broadcasting, in which image transmission is to become the mainstream, the application of a high-efficiency modulation method, such as multilevel QAM, will be essential. Thus, there is an increasing demand for an improvement in the receiving characteristics in the OFDM transmission method, in particular, for the development of an OFDM diversity receiving apparatus that exhibits good receiving characteristics even in response to frequency selective fading.
It is therefore an object of the present invention to provide an OFDM diversity receiving apparatus for use in an OFDM radio communication/broadcasting system that suppresses the degradation of the receiving characteristics under a multiple reflection wave propagation environment or under a moving reception environment.
To achieve the above object, the present invention provides a diversity receiving apparatus for use in a radio communication system that uses an OFDM technique. The diversity receiving apparatus includes a plurality of diversity branches. Each of the diversity branches has a receiver for receiving an OFDM signal via an antenna and outputting the OFDM received signal, a first transformer for transforming the OFDM received signal into a frequency spectrum and outputting the OFDM received signal frequency spectrum, and a channel frequency response calculating unit for calculating a channel frequency response in accordance with the OFDM received signal frequency spectrum and a reference frequency spectrum. A selector unit selects the diversity branch that has generated the OFDM received signal frequency spectrum having a maximum amplitude or a maximum power.
The present invention also provides a diversity receiving apparatus for use in a radio communication system that uses an OFDM technique. The diversity receiving apparatus includes a plurality of diversity branches. Each of the diversity branches has a receiver for receiving an OFDM signal via an antenna and outputting the OFDM received signal, a first transformer for transforming the OFDM received signal into a frequency spectrum and outputting the OFDM received signal frequency spectrum, and a channel frequency response calculating unit for calculating a channel frequency response in accordance with the OFDM received signal frequency spectrum and a reference frequency spectrum. A selector unit selects the diversity branch that has generated the channel frequency response having a maximum amplitude or a maximum power.
More specifically, a first aspect of the present invention provides a diversity receiving apparatus for use in a radio communication system that uses an OFDM technique. The diversity receiving apparatus includes a plurality of receivers, each of the receivers for receiving an OFDM signal via an antenna and outputting the OFDM received signal. Each of a plurality of first transformers transforms the OFDM received signal into a frequency spectrum. At least one reference frequency spectrum generator generates a reference frequency spectrum relative to the OFDM received signal. Each of a plurality of channel frequency response calculating units calculates a channel frequency response in accordance with the frequency spectrum output from each of the first transformers and the reference frequency spectrum. A distortion compensator compensates for a distortion of the frequency spectrum output from each of the first transformers by using the corresponding channel frequency response. The frequency spectra output from the first transformers are input into a selector unit as first input signals, while the distortion-compensated frequency spectra output from the distortion compensator are input into the selector unit as second input signals. The selector unit then selects and outputs the second input signal corresponding to the first input signal having a maximum amplitude or a maximum power. A demodulator demodulates the output signal of the selector unit into a digital signal sequence.
A second aspect of the present invention provides a diversity receiving apparatus for use in a radio communication system that uses an OFDM technique. The diversity receiving apparatus includes a plurality of receivers, each of the receivers for receiving an OFDM signal via an antenna and outputting the OFDM received signal. Each of a plurality of first transformers transforms the OFDM received signal into a frequency spectrum. At least one reference frequency spectrum generator generates a reference frequency spectrum relative to the OFDM received signal. Each of a plurality of channel frequency response calculating units calculates a channel frequency response in accordance with the frequency spectrum output from each of the first transformers and the reference frequency spectrum. A distortion compensator compensates for a distortion of the frequency spectrum output from each of the first transformers by using the corresponding channel frequency response. The channel frequency responses calculated by the channel frequency response calculating units are input into a selector unit as first input signals, while the distortion-compensated frequency spectra output from the distortion compensator are input into the selector unit as second input signals. The selector unit then selects and outputs the second input signal corresponding to the first input signal having a maximum amplitude or a maximum power. A demodulator demodulates the output signal of the selector unit into a digital signal sequence.
A third aspect of the present invention provides a diversity receiving apparatus for use in a radio communication system that uses an OFDM technique. The diversity receiving apparatus includes a plurality of receivers, each of the receivers for receiving an OFDM signal via an antenna and outputting the OFDM received signal. Each of a plurality of first transformers transforms the OFDM received signal into a frequency spectrum. At least one reference frequency spectrum generator generates a reference frequency spectrum relative to the OFDM received signal. Each of a plurality of channel frequency response calculating units calculates a channel frequency response in accordance with the frequency spectrum output from each of the first transformers and the reference frequency spectrum. A distortion compensator compensates for a distortion of the frequency spectrum output from each of the first transformers by using the corresponding channel frequency response. A selector unit selects and outputs, among the distortion-compensated frequency spectra output from the distortion compensator, the distortion-compensated frequency spectrum having a maximum amplitude or a maximum power. A demodulator demodulates the output signal of the selector unit into a digital signal sequence.
Upon generation of frequency selective fading, the receiving characteristics are completely different according to the line spectrum which forms a frequency spectrum. Even under this environment, according to the first, second, and third aspects of the present invention, diversity reception that obtains optimal receiving characteristics for each line spectrum can be achieved. It is thus possible to effectively suppress the degradation of the receiving characteristics caused under a multiple reflection propagation environment or a mobile receiving environment.
A fourth aspect of the present invention provides a diversity receiving apparatus for use in a radio communication system that uses an OFDM technique. The diversity receiving apparatus includes a plurality of receivers, each of the receivers for receiving an OFDM signal via an antenna and outputting the OFDM received signal. Each of a plurality of first transformers transforms the OFDM received signal into a frequency spectrum. At least one reference frequency spectrum generator generates a reference frequency spectrum relative to the OFDM received signal. Each of a plurality of channel frequency response calculating units calculates a channel frequency response in accordance with the frequency spectrum output from each of the first transformers and the reference frequency spectrum. The frequency spectra output from the first transformers are input into a selector unit as first input signals, while the channel frequency responses calculated by the channel frequency response calculating units are input into the selector unit as second input signals. The selector unit then selects the first input signal having a maximum amplitude or a maximum power and the second input signal corresponding to the selected first input signal. The selector unit further outputs the selected first input signal and the selected second input signal as a first output signal and a second output signal, respectively. A distortion compensator compensates for a distortion of the first output signal of the selector unit by using the second output signal of the selector unit. A demodulator demodulates the output signal of the distortion compensator into a digital signal sequence.
A fifth aspect of the present invention provides a diversity receiving apparatus for use in a radio communication system that uses an OFDM technique. The diversity receiving apparatus includes a plurality of receivers, each of the receivers for receiving an OFDM signal via an antenna and outputting the OFDM received signal. Each of a plurality of first transformers transforms the OFDM received signal into a frequency spectrum. At least one reference frequency spectrum generator generates a reference frequency spectrum relative to the OFDM received signal. Each of a plurality of channel frequency response calculating units calculates a channel frequency response in accordance with the frequency spectrum output from each of the first transformers and the reference frequency spectrum. The channel frequency responses calculated by the channel frequency response calculating units are input into a selector unit as first input signals, while the frequency spectra output from the first transformers are input into the selector unit as second input signals. The selector unit then selects the first input signal having a maximum amplitude or a maximum power and the second input signal corresponding to the selected first input signal. The selector unit further outputs the selected first input signal and the selected second input signal as a first output signal and a second output signal, respectively. A distortion compensator compensates for a distortion of the second output signal of the selector unit by using the first output signal of the selector unit. A demodulator demodulates the output signal of the distortion compensator into a digital signal sequence.
Upon generation of frequency selective fading, the receiving characteristics are completely different according to the line spectrum which forms a frequency spectrum. Even under this environment, according to the first, second, and third aspects of the present invention, diversity reception that obtains optimal receiving characteristics for each line spectrum can be achieved. It is thus possible to effectively inhibit the degradation of the receiving characteristics caused under a multiple reflection propagation environment or a mobile receiving environment. Additionally, only a single distortion compensator is required.
According to the first, second, or third aspect of the present invention, a filtering unit for filtering the channel frequency response calculated by each of the channel frequency response calculating units may be disposed between each of the channel frequency response calculating units and the distortion compensator. Similarly, according to the fourth or fifth aspect of the present invention, a filtering unit for filtering the channel frequency response calculated by each of the channel frequency response calculating units may be disposed between each of the channel frequency response calculating units and the selector unit. By the provision of the filtering unit, noise components contained in the channel frequency responses can be eliminated, thereby further enhancing the receiving characteristics.
The above filtering unit may include a filtering-bandwidth-variable filter that receives the channel frequency response calculated by each of the channel frequency response calculating units, a second transformer for transforming the channel frequency response into a channel impulse response, a propagation delay period measuring portion for measuring, based on the channel impulse response, the propagation delay period caused under a multiple reflection propagation environment, and a filtering bandwidth setting portion for setting the filtering bandwidth of the filter based on the measurements by the propagation delay period measuring portion. With this arrangement, the propagation delay period of a multiple reflection propagation environment can be measured, thereby setting the bandwidth of the filter in response to the propagation delay period. As a result, noise components included in the channel frequency response of the propagation delay period can be efficiently eliminated.
The present invention may further include a re-modulator for re-modulating the digital signal sequence demodulated by the demodulator and for generating a re-modulated frequency spectrum, and a frequency spectrum selector for selectively outputting the re-modulated frequency spectrum or the reference frequency spectrum to the channel frequency response calculating units.
In a communication system or a broadcasting system in which digital signal sequences are transmitted according to an OFDM transmission method using a slot, the frequency spectrum selector may select the reference frequency spectrum when receiving an OFDM signal of a known data sequence contained in the head of the slot. When receiving the OFDM signals of the other data sequences, the frequency spectrum selector may select the re-modulated frequency spectrum. Upon calculating the channel frequency response by using the re-modulated frequency spectrum, the distortions of the frequency spectrum can be compensated by using the most updated channel frequency response. It is thus possible to improve the receiving characteristics even under a dynamic propagation environment.
According to the present invention, the selector unit may include a plurality of synthesizers for synthesizing the amplitudes or the power of line spectra forming the first input signal, and a comparator for comparing outputs of the synthesizers, thereby performing a selecting operation based on a comparison result of the comparator. With this arrangement, the diversity branch that exhibits excellent receiving characteristics can be easily selected, thereby effectively enhancing the receiving characteristics.
Alternatively, the selector unit may include a comparator for comparing, among line spectra forming the first input signal, the amplitude or the power of the line spectra of the same frequency, thereby performing a selecting operation based on a comparison result of the comparator. This makes it possible to easily select the diversity branch that exhibits excellent receiving characteristics for each line spectrum, thereby improving the receiving characteristics even more effectively.
A sixth aspect of the present invention provides a method for receiving a diversity signal for use in a radio communication system that uses an OFDM technique. The method includes the steps of: providing a plurality of diversity branches, each of the diversity branches including a receiver for receiving an OFDM signal via an antenna and outputting the OFDM received signal, a first transformer for transforming the OFDM received signal into a frequency spectrum and outputting the OFDM received signal frequency spectrum, and a channel frequency response calculating unit for calculating a channel frequency response in accordance with the OFDM received signal frequency spectrum and a reference frequency spectrum; and selecting the diversity branch that has generated the OFDM received signal frequency spectrum having a maximum amplitude or a maximum power.
A seventh aspect of the present invention provides a method for receiving a diversity signal for use in a radio communication system that uses an OFDM technique. The method includes the steps of: providing a plurality of diversity branches, each of the diversity branches including a receiver for receiving an OFDM signal via an antenna and outputting the OFDM received signal, a first transformer for transforming the OFDM received signal into a frequency spectrum and outputting the OFDM received signal frequency spectrum, and a channel frequency response calculating unit for calculating a channel frequency response in accordance with the OFDM received signal frequency spectrum and a reference frequency spectrum; and selecting the diversity branch that has generated the channel frequency response having a maximum amplitude or a maximum power.
An eighth aspect of the present invention provides a method for receiving a diversity signal for use in a radio communication system that uses an OFDM technique. The method includes the steps of receiving an OFDM signal via an antenna and outputting the OFDM received signal; transforming the OFDM received signal into a frequency spectrum; generating a reference frequency spectrum relative to the OFDM received signal; calculating a channel frequency response in accordance with the frequency spectrum and the reference frequency spectrum; compensating for a distortion of the frequency spectrum by using the corresponding channel frequency response; selecting and outputting, if the frequency spectra are determined as first input signals and the distortion-compensated frequency spectra are determined as second input signals, the second input signal corresponding to the first input signal having a maximum amplitude or a maximum power; and demodulating the output signal of the selecting step into a digital signal sequence.
A ninth aspect of the present invention, there is provided a method for receiving a diversity signal for use in a radio communication system that uses an OFDM technique. The method includes the steps of receiving an OFDM signal via an antenna and outputting the OFDM received signal; transforming the OFDM received signal into a frequency spectrum; generating a reference frequency spectrum relative to the OFDM received signal; calculating a channel frequency response in accordance with the frequency spectrum and the reference frequency spectrum; compensating for a distortion of the frequency spectrum by using the corresponding channel frequency response; selecting and outputting, if the channel frequency responses are determined as first input signals and the distortion-compensated frequency spectra are determined as second input signals, the second input signal corresponding to the first input signal having a maximum amplitude or a maximum power; and demodulating the output signal of the selecting step into a digital signal sequence.
A tenth aspect of the present invention provides a method for receiving a diversity signal for use in a radio communication system that uses an OFDM technique. The method includes the steps of receiving an OFDM signal via an antenna and outputting the OFDM received signal; transforming the OFDM received signal into a frequency spectrum; generating a reference frequency spectrum relative to the OFDM received signal; calculating a channel frequency response in accordance with the frequency spectrum and the reference frequency spectrum; compensating for a distortion of the frequency spectrum by using the corresponding channel frequency response; selecting and outputting, among the distortion-compensated frequency spectra, the distortion-compensated frequency spectrum having a maximum amplitude or a maximum power; and demodulating the output signal of the selecting step into a digital signal sequence.
An eleventh aspect of the present invention provides a method for receiving a diversity signal for use in a radio communication system that uses an OFDM technique. The method includes the steps of receiving an OFDM signal via an antenna and outputting the OFDM received signal; transforming the OFDM received signal into a frequency spectrum; generating a reference frequency spectrum relative to the OFDM received signal; calculating a channel frequency response in accordance with the frequency spectrum and the reference frequency spectrum; selecting, if the frequency spectra are determined as first input signals and the channel frequency responses are determined as second input signals, the first input signal having a maximum amplitude or a maximum power and the second input signal corresponding to the selected first input signal, and outputting the selected first input signal and the selected second input signal as a first output signal and a second output signal, respectively; compensating for a distortion of the first output signal by using the second output signal; and demodulating the output signal of the compensating step into a digital signal sequence.
A twelfth aspect of the present invention provides a method for receiving a diversity signal for use in a radio communication system that uses an OFDM technique. The method includes the steps of receiving an OFDM signal via an antenna and outputting the OFDM received signal; transforming the OFDM received signal into a frequency spectrum; generating a reference frequency spectrum relative to the OFDM received signal; calculating a channel frequency response in accordance with the frequency spectrum and the reference frequency spectrum; selecting, if the channel frequency responses are determined as first input signals and the frequency spectra are determined as second input signals, the first input signal having a maximum amplitude or a maximum power and the second input signal corresponding to the selected first input signal, and outputting the selected first input signal and the selected second input signal as a first output signal and a second output signal, respectively; compensating for a distortion of the second output signal by using the first output signal; and demodulating the output signal of the compensating step into a digital signal sequence.